I posted an earlier prototype with cruder wheels - this is the same frame and box with the production wheels on it. I also included a close up picture of the hitch/joint assembly. This one has worked fairly well although it's vulnerable to twist. The production version will replace the twisted metal strap piece with two pieces of angle iron bolted together with a nylon washer between them. The clamp has a piece of hardwood with a notch cut into it to maintain orientation. The production model will also have a clamping bolt that's threaded all along it's length so I can dispense with the washers. A nicer T-knob will replace the wingnut